Official Answer
Answer
This phenomenon is known as 'Set' or 'Relaxation'. If the spring is compressed to a height where the internal stresses exceed the material's elastic limit, plastic deformation occurs. For a new design, engineers should perform a 'Preset' operation where the spring is compressed to its solid height (or maximum work height) during manufacturing. This 'cold works' the crests and induces beneficial compressive residual stresses. If a spring loses height in the field, it indicates the operating stress $S$ was underestimated or the operating temperature exceeded the material's limits.
